The present invention relates to a thermogravimetric instrument for measuring changes in the weight of a sample caused by temperature changes.
A differential thermo-balance having a horizontally placed balance beam and available as a conventional instrument of this kind is described in Japanese Patent No. SHO 62-4654 (issued Jan. 31, 1987). In this instrument, a sample holder for holding both a sample and a sample container is spaced from a pivot, which in turn holds the balance horizontally and in a tiltable manner. Although not mentioned in the prior literature, a torsion wire mounted as a pivot for the balance beam acts to sustain the weight of the balance beam and to produce a rotation moment within a plane vertical to the axis because of a quite small torsional rigidity for detection of slight changes in the weight of the sample.
With this instrument, if the sample boils over or the sample container is fused and bonded to the beam to thereby make the instrument unusable, it is necessary to detach the balance beam. In this case, the balance beam is replaced by a new one by detaching the old balance beam at the position of a junction portion at the midway position of the balance beam. The junction portion permits mounting a sample holder side balance beam. The old balance beam is detached at the location of the junction portion. Then, the old beam is replaced with a new one.
Where the balance beam should be detached, the following method may also be adopted. A part of the balance beam is anchored with a clamp or the like to prevent application of any load to the pivot. The balance beam is then disconnected from the junction portion. Thereafter, the balance beam is detached.
With the aforementioned configuration, the following problems take place when the balance beam is detached.
In order to enhance the weight sensitivity of the balance, the above-described balance beam and torsion wire are so constructed as to minimize the weight of the balance beam and the torsional rigidity of the torsion wire. Although the mechanical rigidity of the torsion wire is reduced to improve the weight sensitivity of the balance, the mechanical rigidity must high enough to withstand mechanical loads as encountered during replacement of the sample container on the sample holder or during normal measurement operations. That is, the actual torsion wire should be high enough to stand up to mechanical loads encountered when normal measurement operations are performed.
In order to make replaceable the sample container side of the above-described balance beam, the junction portion is mounted to the balance beam, pushed, pulled, rotated, or otherwise operated to permit detachment. Where the balance beam is detached at the location of the junction portion, a mechanical load is imposed on the torsion wire acting as the pivot of the balance beam. This mechanical load must be large enough to enable the detachment. The minimum force necessary for the detachment is an external force necessary to cause the balance beam to make a transition from a stationary state in which the junction portion is coupled to the balance beam to a disconnected state in which the junction portion is disconnected from the balance beam. The minimum force described above is sufficiently larger than mechanical loads applied to the torsion wire during normal operations, because disengagement of the junction portion during normal operations such as replacement of the sample container should be avoided.
Accordingly, when the above-described balance beam is detached, the mechanical load applied to the torsion wire exceeds the strength of the torsion wire fabricated to have appropriate strength, thus producing undesirable results.
To prevent the torsion wire from being damaged, it is necessary to increase the strength to a level so that the wire can withstand the minimum force necessary for the detachment. If the strength is increased, the balance sensitivity is deteriorated greatly, thus impairing the performance of the instrument.
In some structure, the movement of the balance beam is constrained by clamping or otherwise holding a portion that is closer to the pivot of the balance than the junction portion of the balance beam. Thus, the load applied during detachment of the balance beam is prevented from being transmitted to the pivot of the balance.
In this case, however, the clamp must be detached during normal measurement. Whenever the balance beam is replaced, the beam must be clamped, thus creating an inconvenient situation.
If the sample holder and the balance beam are erroneously detached without clamping or otherwise holding the balance beam, a large mechanical load is applied to the torsion wire that is mounted as the pivot of the balance beam. There will be danger of breakage or damage.
If the relative positional relation between the balance beam and the holding clamp deviates for some reason or other, the position of the balance beam differs before and after the clamping operation. Rather, the clamping operation exerts a large mechanical load on the torsion wire of the pivotal portion of the balance beam with undesirable results.
Accordingly, it is an object of the present invention to provide a mechanism for preventing application of a large mechanical load to a torsion wire without increasing the strength of the torsion wire acting as the pivot of a balance, without lowering the weight sensitivity of the balance, and without performing any special clamping operation during replacement of the balance beam. That is, relative movement between the supported portion of the torsion wire and the balance beam is restricted to within a prescribed amount to prevent the torsion wire from deforming beyond its resilient deformation region.